December 3, 2002
12:00 AM (EST)

News Release Number: STScI-2002-27

Hubble Makes Precise Measure of Extrasolar World's True Mass

December 3, 2002: An international team of astronomers used the Hubble Space Telescope to help make a precise measurement of the mass of a planet outside our solar system. The Hubble results show that the planet is 1.89 to 2.4 times as massive as Jupiter, our solar system's largest orbiting body. Previous estimates, about which there are some uncertainties, place the planet's mass at a much wider range: between 1.9 and 100 times that of Jupiter's. The planet, called Gliese 876b, orbits the star Gliese 876. It is only the second planet outside our solar system for which astronomers have determined a precise mass.

Q & A: Understanding the Discovery

1.
How did Hubble make the measurements of the planet's mass?

Hubble did not view the planet directly. Astronomers used a set of instruments on Hubble called Fine Guidance Sensors to measure a small "side-to-side" wobble of the star due to the tug of the unseen planet. (The Fine Guidance Sensors also are used to point and stabilize the free-flying observatory.) This technique, called astrometry, had never been used to measure the mass of a planet beyond our solar system. Astronomers using ground-based telescopes first discovered the planet in 1998.

The team of astronomers observed the star's "yo-yo" motion for more than two years. Making measurements of a star's movement is like measuring the size of a quarter 3,000 miles away.

The team then combined the Hubble information with previous measurements made by ground-based telescopes using a different technique. That technique, called the radial-velocity method, measured the star's subtle "to-and-fro" speed, caused by the unseen planet's gravitational tug.

Those independent measurements helped the team deduce the planet's orbital inclination toward Earth and thereby calculate the planet's mass. If astronomers don't know how a planet's orbit is tilted toward Earth, they can only estimate its minimum mass. The planet's orbit turns out to be tilted nearly edge-on to Earth, verifying that it is a low-mass object.